Preservation of wood

ABSTRACT

Wood is preserved with a water-based treating solution containing a halogenated phenol wood preservative such as pentachlorophenol, a phenolic resin prepolymer such as a phenol formaldehyde prepolymer, and a coupling or solubilizing agent for enhancing the water compatibility of the various ingredients. The treatment is carried out by exposing the wood, while submerged in the treating solution, so one or more cycles of reduced pressure (i.e. vacuum) and elevated pressure.

This is a division, of application Ser. No. 06/295,850, filed Aug. 24,1981, now U.S. Pat. No. 4,399,195; patented Aug. 16, 1983.

FIELD OF THE INVENTION

This invention relates to the preservation of wood, and includes thechemical compositions used to treat the wood, the process of treatingthe wood, and the treated wood that results from the process. Thechemical compositions which are described herein are especially wellsuited for use in the method of U.S. Pat. No. 3,968,276 in which wood tobe treated is exposed to a reduced pressure for the purpose of removingmoisture and volatiles from the wood, and thereafter impregnating thewood with an aqueous chemical composition.

BACKGROUND OF THE INVENTION

Wood is one of the oldest materials known to man. Its ready availabilityin most parts of the world, coupled with the ease with which it can becut and fabricated into useful articles makes it well suited for avariety of applications. However, because wood is a cellulosic materialit is susceptible to decay and deterioration over any extended period oftime. As a result, numerous methods have been proposed for treating woodto improve its durability, particularly when used under conditions wherethe wood is exposed to moisture or biological attack.

Applications of wood such as its use in manufacturing railroad ties,utility poles, fence posts, and the like have created substantialincentives for industry to develop effective and commercially practicalmethods of treating the wood to extend its useful life (i.e. woodpreservation). Various techniques of wood preservation have been usedincluding surface coating, surface treating, and pressure penetration ofthe wood with a variety of chemicals such as creosote,pentachlorophenol, and the like.

For example, halogenated phenols such as pentachlorophenol are oftenused to treat wooden poles (such as utility poles). When used for thispurpose, it is common to dissolve the pentachlorophenol in a hydrocarbonoil, pressure treat the poles, and then evaporate the excess oil. Such aprocess is relatively expensive, but is justified by the woodpreservative properties of the pentachlorophenol. However, the treatedpoles tend to "bleed" their treating solution into the surrounding soilwhen the poles are installed in the ground. This draining or leachingeffect is more visible on the sunny side of utility poles which drain orbleed faster because the heat from the sun warms the treating solutionand reduces its viscosity. In an effort to reduce treating costs, wateris sometimes substituted for the hydrocarbon oil, and thepentachlorophenol is used in one of its salt forms to provide watersolubility. However, this approach has a number of disadvantages and isless preferred than the oil treatment.

Unfortunately, most of the present treating processes (e.g. those usingcreosote, various copper and mercury compounds, or pentachlorophenol),even when operated under significant pressure conditions, do not resultin deep impregnation of the wood. Thus, the wood being treated with anychemical is typically treated to only a shallow depth and, in the caseof logs or poles very often the degree of penetration does not extendthroughout all of the soft or sapwood, much less penetrate theheartwood. Except for one known process, none of the known commercialprocesses for preserving wood by chemical treatment involves penetratingthe wood poles or logs more than perhaps a few millimeters into theheartwood. The one known exception is the process of U.S. Pat. No.3,968,276.

According to U.S. Pat. No. 3,968,276 wood (logs or dimension lumber) ispreserved by impregnating the wood with chemicals which penetratesubstantially to the core or center of the wood being treated. In thecase of logs or poles, this penetration can be deep within the heartwoodand may be substantially complete. This is accomplished by enclosing thewood within a chamber, sealing the chamber, evacuating the chamberslowly so that the wood is exposed to a pressure below atmosphericpressure, introducing into the chamber a sufficient amount of an aqueoussolution of resin prepolymer to immerse the wood and then maintainingthe vacuum over the immersed wood for a period of time both to allow theremoval of moisture and volatiles from the wood and to permit thetreating solution to enter into the wood. Thereafter, an additionalamount of the resin prepolymer solution is introduced into the chamber(if needed) and the pressure on the system is increased tosuperatmospheric pressure to thereby cause an even deeper penetration ofthe treating solution into the wood. The vacuum and pressure cycle maybe repeated one or more times (e.g. 1 to 30 or more cycles). When thetreatment is completed to the desired extent, the treated wood isrecovered from the chamber.

The process just described is extremely effective and achievessubstantially greater penetration of the wood than other commonlypracticed treating processes.

It would be beneficial to industry if common wood preservatives such aspentachlorophenol could be introduced deep within the wood being treatedand if the bleeding or leaching of the pentachlorophenol could bereduced.

SUMMARY OF THE INVENTION

The present invention is based on the discovery that one or more knownwood preservatives, such as the halogenated phenols, can be introduceddeep into the wood being treated and obtain reduced "bleeding" orleaching, by combining the wood preservative with the method of U.S.Pat. No. 3,968,276 by using a special class of treating compositionsthat are water based or water dilutable which permits the benefits ofthe aqueous resin prepolymer system of U.S. Pat. No. 3,968,276 to bemaintained. Further, it has been discovered that it is possible toaccomplish this without encountering any significant amount of"blooming" which is normally associated with the use of aqueous treatingsolutions of halogenated phenols, such as pentachlorophenol in thetreatment of wood. This blooming effect is caused by the fact that theaqueous treating solutions are made from salts of halogenated phenols(such as the salts of pentachlorophenol) because the salts dissolvereadily in water. As a result, after the phenate salts have been used totreat wood, there is a tendency for the salts to be redissolved by rainwater or ground water and to migrate easily to the surface of thetreated wood where they crystallize during the subsequent evaporation ofthe water.

Briefly described, the present invention involves manufacturing a waterdilutable concentrate or a water based treating solution which consistsessentially of:

(a) a halogenated phenol wood preservative such as pentachlorophenol;

(b) a phenolic resin prepolymer such as a phenol formaldehydeprepolymer;

(c) a coupling or solubilizing agent for enhancing the watercompatibilty of the concentrate (e.g. methyl alcohol or acetone with orwithout the addition of a surfactant); and

(d) water.

Optionally and preferably, the treating solutions will also containorgano/metallic salts, especially copper or zinc salts.

In addition, it has been discovered that the ability of the prepolymerto cure rapidly and lock in the halogenated phenol deep within the woodcan be enhanced by heating the wood at the conclusion of the treatment(e.g. heating the wood with steam) to thereby accelerate the curing ofthe resin prepolymer.

DETAILED DISCUSSION

The present invention concerns water based solutions for treating woodand the use of such solutions to treat wood (both dried wood and greenwood), and the resulting treated wood products. For convenience, it issometimes convenient to make the treating solutions in two steps, thefirst step being the preparation of water-dilutable concentrate orsyrup, and the second step consisting of diluting the concentrate withwater to achieve a desired concentration of active ingredients. Althoughany distinction between the concentrate and the aqueous treatingsolutions, per se, is to a certain extent arbitrary, the treatingsolutions will normally contain from 1-40% (e.g. 2-30%) by weight of thecombined amount of the halogenated phenol wood preservative and aphenolic resin prepolymer, more usually about 5-25% on the same basis.By comparison, the water dilutable concentrates will usually containfrom 10-65% by weight of the combined amount of halogenated phenol woodpreservative and phenolic resin prepolymer, usually within the range of15-50% on the same basis. Of course, the use solutions may be prepareddirectly.

The ingredients which are used in preparing the aqueous treatingsolutions of the present invention and the corresponding water dilutableconcentrates are as follows.

The first essential ingredient is a halogenated phenol woodpreservative. Typically, the halogenated phenol wood preservative willbe a chlorinated phenol. Pentachlorophenol is preferred. Commercialgrade pentachlorophenol (which contains related compounds such astetrachlorophenol) is particularly useful. These halogenated phenol woodpreservatives will ordinarily be used in their phenol form and not as asalt as is conventionally done in the case of treating wood withpentachlorophenol where the phenol is used as the phenate salt becauseit dissolves readily in inexpensive polar solvents such as water.

The resin prepolymer used in the present compositions will be a watercompatible phenolic resin prepolymer, preferably a water based phenolformaldehyde prepolymer. Water soluble or dispersible phenolic resinprepolymers are well known and their preparation, per se, forms no partof this invention. However, all phenolic resin prepolymers do not workwith equal effectiveness, and the aqueous phenolformaldehyde prepolymersare preferred. Low viscosity prepolymers are preferred for ease of use.Such prepolymers typically have molecular weights (weight average) ofless than 1000 (e.g. 80 to 500).

Although the ratio of halogenated phenol wood preservative to phenolicresin prepolymer will vary depending upon a variety of factors includingthe particular ingredients being used, the purpose of the woodtreatment, the type of wood being treated, etc., in general the weightratio of halogenated phenol to resin prepolymer will be within the rangeof from 3:1 to 1:15, and more preferably within the range of 1:2 to1:10.

The treating compositions of the present invention (as well as theconcentrates) will also contain a coupling agent or solubilizing agentfor enhancing the water compatibility of the mixture of halogenatedphenol and resin prepolymer. This coupling agent will typically comprisea polar co-solvent such as alcohols and ketones. Alcohols such as C₁ -C₄alkanols (e.g. methyl alcohol) are preferred, with or without addedsurfactants (e.g. surfactants of the polyethoxy type). The amount ofsurfactant will usually be 0 to 5 weight-% (e.g. 0.1 to 3 weight-%),based on the total composition weight. The amount of coupling agent usedwill depend on the coupling agent that is used and the amount ofhalognated phenol present.

The treating compositions of the present invention (as well as theconcentrates) will also contain water as an essential ingredient. In thedilute or use solutions, the water content will generally be at least50% by weight of the treating solution, usually 55-95% by weight (e.g.60-75% by weight). In the case of the concentrates, water will normallybe present in amounts of 50% by weight or more of the weight of thecoupling agent (e.g. methanol with or without added surfactants).

If desired, the treating compositions of the present invention and theconcentrates may contain other ingredients such as fire retardantchemicals, dyes or colorants, catalysts, various biocides, and otherchemicals (e.g. borax which serves as a buffer or stabilizer). Thepresence of other wood preservatives such as the organo-metalliccompounds is optional but preferred. Compounds of zinc and copper areparticularly desirable. Suitable organo/metallic compounds include zincnaphthenate, copper naphthenate and copper-8-quinolinolate.

To prepare the treating compositions of the present invention (and theconcentrates), the preferred procedure is to dissolve the halogenatedphenol (e.g. pentachlorophenol) in the alcohol, or other coupling agent,and then slowly adding to the resulting mixture an aqueous resinprepolymer, (usually with vigorous agitation), then adding thesurfactant (if one is used), then the metallic compounds (if any), andthen finally adding such additional water as may be appropriate, notingthat the total amount of water to be added will vary depending upon thedesired strength of the final product and depending upon whether or notone is interested in producing only a concentrate for convenientshipment to a distant point of usage or whether one wishes to preparethe final composition at its desired level of use.

Optional ingredients can be added at any convenient point. If desired,the addition of the various ingredients can be done simultaneously,sequentially, or incrementally. However, the general procedure as justoutlined is preferred, i.e. addition of the halogenated phenol to thealcohol or other coupling agent, followed by addition of the resinprepolymer and subsequent dilution with water. Borax and other bufferscan be added last.

In treating wood, the compositions of the type just described will beprepared at an appropriate solids content level (.e. the appropriatelevel of dilution) and used according to the following general steps:

1. Wood to be treated is selected for treatment. The wood will bedebarked and usually precut (e.g. dimension lumber) or preshaped (e.g.poles or fence posts). It may be green or air-dried or force-dried andwill often be in the form of poles or dimension lumber. The ability totreat green wood (i.e. wood containing a significant amount of naturalmoisture) is an important advantage of the present process.

2. The wood to be treated is placed in an evacuation zone and exposed topressures which are below atmospheric pressure (e.g. at a vacuum of atleast 20 inches of mercury, preferably at least 26 inches of mercury)for a time sufficient to remove moisture and other volatiles from thewood. Vacuums of at least 28 inches of mercury are preferred. Thisreduced pressure is usually achieved slowly over a period of from 10 to120 minutes (e.g. 15 to 60 minutes).

3. The wood is then immersed in or flooded with the treating compositionwhile still under reduced pressure. During this phase of the treating,additional water and volatiles escape from the wood and the treatingsolution begins to penetrate into the wood.

4. The pressure within the treating chamber or zone is then graduallyincreased to a level above atmospheric pressure (e.g. often to a levelof 25 psig-500 psig, usually 50 to 350 psig) to thereby increase thepenetration of the wood by the treating agent. Typically, this step iscontinued until the treating agent has substantially completelypenetrated into the heartwood, although the process could of course bestopped short of that point if a lesser treatment is desired.

5. To further improve and enhance the depth and quality of penetrationof the treating solution into the wood, the vacuum and pressure cyclecan be repeated as many times as desired (e.g. 2 to 25 or more cycles).When the process is cycled in this fashion, it is helpful if the maximumpressure during each cycle is increased each time by, for example,10-100 psig above the maximum pressure of the previous cycle (usually apressure increase of 25% to 100% over the previous pressure). Thisappears to prevent the rupture and collapse of the wood cells near thesurface of the wood.

6. Recovering treated wood from the treating chamber.

Note that the vacuum and pressure phases of the treating cycle should beachieved gradually to avoid destruction of the cell walls of the wood.Thus, while speed is important for commercial purposes (e.g. output andeconomics), too rapid a change in pressure can be harmful. Consequently,the optimum cycle rates, pressure and vacuum should be determined foreach set of operating conditions to be used (e.g. size, shape and typeof wood to be treated; composition of the treating compositions;temperature; etc.).

Since it is the intention of the present process to cause or permit theresin prepolymer to cure after it has penetrated deep within the wood,the conditions of treating including the choice of resin prepolymershould be made so as to permit easy penetration of the wood followed bya rather rapid curing of the prepolymer after penetration. Although avariety of techniques including the use of latent catalysts might beused to accelerate the curing of the prepolymer, it has been discoveredthat one very convenient method of accelerating the curing of theprepolymer after the treating solution has deeply penetrated into thewood is to heat the treated wood. Although this heating can beaccomplished in a variety of ways, the direct application of steam tothe treated wood is preferred. This method requires no special chemicalsand avoids the probems of premature curing that are sometimes associatedwith the use of catalysts. Moreover, the use of steam is convenient,economical, environmentally acceptable, and does not cause any clean-upproblems.

The present process is likewise non-polluting and does not producehazardous by-products. When the pre-polymer has cured in place, itserves to lock in the halogenated phenol and other optional ingredients,retard or prevent leaching or bleeding of the halogenated phenol andother optional ingredients (e.g. the organo-metallic compounds), andstrengthens the wood. It is theorized that the pre-polymer reacts withhydroxyl groups present in the cellulosic chains of the wood. If so,this may account for why the treated wood becomes so resistant tomicrobiological deterioration and insect attack.

The present invention will be further understood by reference to thefollowing Examples that include a preferred embodiment. Unless otherwiseindicated, all parts and percentages are by weight.

Example 1

Ten parts of pentachlorophenol (commercial grade including the normalimpurities) was mixed with 35 parts of methyl alcohol. Next, 20 parts byweight of an aqueous phenol formaldehyde prepolymer (Hooker resin 29765having a molecular weight [weight average] of 141.9; a product of HookerChemical Company) is slowly added to the mixture. After the addition iscompleted, the liquid concentrate is further diluted with 35 parts ofwater.

EXAMPLE 2

The procedure of Example 1 is repeated. However, one part of a nonionicsurfactant (Triton N-57, product of Rohm & Haas Company) is added to themixture of methyl alcohol and pentachlorophenol prior to diluting thatmixture with water.

EXAMPLE 3

The procedure of Example 2 is repeated with the further addition ofone-half part of a 10 mole solution of borax to the completed mixture.

EXAMPLES 4-7

In Table 1 which follows are shown the approximate formulas for severaltreating compositions which may be made according to the presentinvention following the general procedure of Example 1.

                  TABLE 1                                                         ______________________________________                                                                               Ex-                                                  Example  Example  Example                                                                              ample                                  Ingredient    4        5        6      7                                      ______________________________________                                        (a) resin prepolymer                                                                            7.5      4.3    10     5                                        solids.sup.1                                                              (b) pentachlorophenol                                                                           5.5      6.6    3      2                                        (commercial grade)                                                        (c) methanol      19.0     25.6   15     20                                   (d) water         68.0     61.8   71.7   67.5                                 (e) surfactant.sup.2                                                                            --       1.0    0.25   1.5                                  (f) borax.sup.3   --       0.7    0.025  --                                   (g) copper-8-quinoli-                                                                           --       --     --     4                                        nolate solids                                                             ______________________________________                                         .sup.1 From an aqueous phenol formaldehyde prepolymer having a solids         content of 70% and a molecular weight (weight average) of 141.9; a produc     of Hooker Chemical Co.                                                        .sup.2 nonylphenoxypolyethoxy ethanol; Triton N57, a product of Rohm &        Haas.                                                                         .sup.3 from a 25% solids solution; a product of Mooney Chemical Co.      

EXAMPLE 8

Wood (Douglas fir) is treated with the treating composition of Example 4by the following method.

The wood is a green Douglas Fir pole 10 inches in diameter. This wood isplaced in a pressure treating vessel and the air is evacuated over aperiod of 20 minutes to a vacuum of 28 inches Hg. This reduced pressureis maintained for 30 minutes to permit water and volatiles in the woodto be removed. Next, the treating solution (Example 4 above) is pumpedinto the treating chamber so as to totally immerse or submerge the woodin the treating solution. The reduced pressure is maintained for anadditional period of 90 minutes.

Thereafter, the pressure is allowed to increase to 25 psig and held for40 minutes. The pressure is slowly released and the vacuum reapplied for10 minutes. The pressure is then raised to 50 psig for 40 minutes. Then,the vacuum is applied for 10 minutes followed by pressure at 75 psig for40 minutes. Then anoher cycle of vacuum for 10 minutes, followed by apressure of 100 psig for 40 minutes. Then the vacuum is reapplied for 10minutes followed by pressure at 150 psig for 8 hours. Then, the vacuumis reapplied for 10 minutes, followed by 240 psig pressure for 60minutes. Then, a final vacuum cycle for 20 minutes. Then the pressure isreturned to atmospheric pressure and the pressure treating vessel isdrained.

After the treating solution is drained from the pressure treatingvessel, but before removing the poles, steam at 15-50 psig is optionallyadded to the vessel for 15-30 minutes (depending on size). The purposeof the steam is to accelerate the curing of the resin prepolymer whichhas penetrated into the wood.

Then, the treated wood is removed from the pressure treating vessel.

What is claimed is:
 1. A method of treating wood, by causing the deeppenetration of the wood by a treating composition, which comprises:(a)placing the wood to be treated in an evacuation zone and exposing thewood to pressures below atmospheric pressure for a time sufficient toremove moisture and other volatiles from the wood; (b) immersing thewood in a water based treating composition consisting essentially of aneffective amount of a halogenated phenol wood preservative, a sufficientamount of a phenolic resin prepolymer which upon curing will preventpreservative migration, and an amount of a coupling agent sufficient tosubstantially maintain the preservative and the prepolymer in solution;(c) maintaining the wood immersed in the treating composition under saidreduced pressure until the solution has penetrated into the wood; (d)increasing the pressure above the liquid to a pressuure aboveatmospheric to thereby increase the penetration of the wood by saidtreating agent; and (e) recovering the treated wood.
 2. Methods of claim1 in which the treated wood is heated to accelerate the curing of theprepolymer.
 3. Methods of claim 2 in which the heating is accomplishedwith steam.
 4. Methods of claim 1 in which the halogenated phenol ispenta-chloro phenol.
 5. Methods of claim 4 in which the resin prepolymeris a phenol formaldehyde prepolymer.
 6. Methods of claim 5 in which theweight ratio of halogenated phenol to resin prepolymer is from 3:1 to1:15.
 7. Methods of claim 6 in which the coupling agent comprises methylalcohol.
 8. Methods of claim 7 in which the treating agent consistsessentially of:(a) penta chloro phenol; (b) phenol formaldehydeprepolymer, the weight ratio of penta chloro phenol to prepolymer beingfrom 1:2 to 1:10; (c) methyl alcohol; (d) at least 50% by weight ofwater; and (e) from 0 to 5% by weight of at least one surfactant. 9.Methods of claim 8 in which the treating agent also containscopper-8-quinolinolate.
 10. Methods of claim 8 in which the wood isgreen and in which steps "c" and "d" are repeated from 1 to 30 times.11. Treated wood produced by the process of claim
 1. 12. Treated woodproduced by the process of claim
 2. 13. Treated wood produced by theprocess of claim
 3. 14. Treated wood produced by the process of claim 4.15. Treated wood produced by the process of claim
 5. 16. Treated woodproduced by the process of claim
 6. 17. Treated wood produced by theprocess of claim
 7. 18. Treated wood produced by the process of claim 8.19. Treated wood produced by the process of claim 9.